The present invention relates to ignition compositions, and more particularly to ignition compositions for inflator gas generators utilized in vehicle occupant restraint systems.
A steel canister is commonly utilized as the inflator pressure vessel for an automobile occupant restraint system because of the relatively high strength of steel at elevated temperatures. However, emphasis on vehicle weight reduction has renewed interest in the use of aluminum in place of steel in such pressure vessels.
One test that vehicle occupant restraint inflator systems must pass is exposure to fire whereupon the gas generating material of the inflator is expected to ignite and burn, but the inflator pressure vessel must not rupture or throw fragments. Steel pressure vessels pass this test relatively easily because steel retains most of its strength at ambient temperatures well above the temperature of which the gas generant autoignites. Aluminum, however, loses strength rapidly with increasing temperature and may not be able to withstand the combination of high ambient temperature and high internal temperature and pressure generated upon ignition of the gas generant. If, however, the gas generant of the inflator can be made to autoignite at relatively low temperatures, for example, 1350.degree. C.(275.degree. F.) to 210.degree. C. (410.degree. F.) the inflator canisters can be made of aluminum.
Providing autoignition compositions for use in aluminum pressure vessels has heretofore been problematic. U.S. Pat. No. 4,561,675 granted to Adams et al, which discloses the use of Dupont 3031 single base smokeless powder as an autoignition gas generant, is exemplary of an unreliable autoignition composition found in the prior art. While such smokeless powder autoignites at approximately the desired temperature of 177.degree. C. (.apprxeq.350.degree. F.), it is largely composed of nitrocellulose. One of ordinary skill in the propellant field will appreciate that nitrocellulose is not stable for long periods at high temperatures, which is a specific requirement in automotive applications.
In addition, commonly assigned U.S. Pat. No. 5,084,118 to Poole, describes other autoignition compositions, which comprise 5-aminotetrazole, potassium or sodium chlorate, and 2,4-dinitrophenylhydrazine. While the compositions disclosed in U.S. Pat. No. 5,084,118 autoignite and cause ignition of the gas generant when heated to approximately 177.degree. C. (.apprxeq.350.degree. F.), the compositions have not proven to be fully satisfactory. The manufacture of these compositions is difficult and hazardous because of the utilization of hexane and xylene in the manufacturing process. Hexane has a low boiling temperature and thus requires careful handling, while xylene is a suspected carcinogen. In addition, the compositions disclosed in U.S. Pat. No. 5,084,118 are not effective after long-term ageing. Vehicle occupant restraint inflator systems must pass ageing requirements in order to ensure reliable ignition despite exposure to a wide range of temperatures over the life of the vehicle.